| AT/PCSA ratio |
Ratio of tendon cross-sectional area to muscle PCSA; sets tendon stiffness relative to the force the muscle can apply. Low AT/PCSA → compliant tendon, high elastic energy cycling. |
| Achilles tendon |
The free distal tendon connecting the triceps surae (gastrocnemius + soleus) to the calcaneus; one of the body’s most important elastic-energy storage elements. Disproportionately long in humans compared to other great apes. |
| Actin (thin filament) |
The thin contractile filament; provides binding sites for myosin heads. Binding sites are blocked by tropomyosin in the absence of Ca2+. |
| Activation level (Fact) |
A scalar (0–1) representing the fraction of fibers active or the level of Ca2+ activation; in standard muscle models, scales the F–L–V surface multiplicatively. |
| Aerial phase |
The portion of the stride cycle when no foot is on the ground (running, hopping). |
| Allometric scaling |
Scaling with body size in which proportions change — e.g., the postural shift that raises EMA in larger mammals. |
| AMPK |
AMP-activated protein kinase; an energy sensor that activates endurance signaling (via PGC-1α) and inhibits mTOR (via TSC1/2) — the molecular basis of concurrent-training interference. |
| Ankle exoskeleton |
A wearable elastic device that adds parallel rotational stiffness to the ankle joint; reduces metabolic cost when its stiffness is matched to the wearer’s biological muscle–tendon dynamics. |
| Antioxidant enzyme capacity |
Enzymatic capacity to neutralize free radicals produced during exercise; can rise by ~100% over 12 weeks of training. |
| Aponeurosis |
A flat, broad internal tendon that lies on (or within) the muscle belly and connects to the external tendon. Part of total tendon length. |
| Atrophy |
Decrease in muscle cell volume in response to detraining, immobilization, or denervation. In the current model, myonuclei are retained even as the cytoplasm shrinks. |
| a-v O2 difference |
Difference between arterial and venous O2 content; rises slowly with peripheral adaptation (capillary density, mitochondrial density) and accounts for most long-term VO2 max gain. |
| B-mode ultrasound |
Non-invasive imaging technique that resolves fascicle length and pennation angle in real time in living humans during movement; the human analog of sonomicrometry. |
| Bone safety factor |
Ratio of bone failure strength to peak bone stress during typical locomotion; about 2–4 across vertebrates, because bone remodels to match the loads it routinely experiences. |
| Bouncing gait |
A locomotion pattern (running, hopping, trotting) in which gravitational and kinetic energies fluctuate in phase and elastic structures cycle the energy. |
| Bramble & Lieberman 2004 |
Foundational paper proposing the endurance-running hypothesis. Catalogs human anatomical features argued to be running-specific; critiques include limited fossil evidence and alternative interpretations as walking adaptations. |
| Brake / energy absorber (muscle function) |
A muscle that lengthens while producing high force, doing negative net work and dissipating energy from the system. |
| Cardiac output (Q) |
Heart rate × stroke volume; rises with training mainly via increased stroke volume and accounts for most of the short-term VO2 max gain. |
| Carotid rete |
A meshwork of arterioles around the carotid artery in many cursorial mammals that cools the blood supply to the brain via heat exchange with cooler nasal venous return. Humans lack a carotid rete. |
| Center of mass (CoM) |
The single point at which the body’s mass can be approximated as concentrated for whole-body dynamics. |
| Center-of-pressure excursion index (CPEI) |
A measure of how much the center of pressure rolls forward along the foot during stance, normalized to foot width. Larger excursion → more efficient walking. |
| Collision |
The energy-dissipating impact at heel strike when the leading leg comes down with downward velocity that must be reversed. |
| Collisional energy loss |
Energy dissipated at the moment of foot-ground contact during step-to-step transitions; a major irreducible energy cost in legged locomotion. |
| Compliant tendon |
A tendon that stretches substantially under physiological loads; stores and returns elastic strain energy. Low AT/PCSA. |
| Concentric contraction |
Shortening contraction in which muscle force exceeds the load; performs positive work; most expensive in ATP per unit force. |
| Concurrent training |
Combined endurance + resistance training; can produce smaller strength gains than resistance alone because endurance signaling (AMPK) inhibits mTOR. |
| Cost coefficient (C) |
Energy used per Newton of body weight supported, per second; ~0.189 J/N, approximately constant across species and speeds. |
| Cost of transport (CoT) |
Energy used per unit distance traveled, per unit body mass (J kg⁻¹ m⁻¹). The central metric for comparing locomotor economy across species and speeds. |
| Cross-bridge cycle |
The six-step molecular cycle of myosin head attachment, power stroke, ADP release, ATP binding, and hydrolysis-driven re-cocking that produces sarcomere shortening. |
| Cross-reinnervation |
Experimental swap of nerves between fast and slow muscles, demonstrating that fiber type can shift in response to the pattern of neural activation. |
| Crouched posture |
The flexed-limb posture seen in small animals (mice, shrews) and in humans during deep knee bends; lower EMA, higher muscle-force demand. |
| Cursorial |
Adapted for sustained terrestrial locomotion; cursorial morphology includes upright parasagittal limbs, elongated distal limbs, reduced distal-limb mass, and elaborated tendons. |
| Detraining |
Decline in fitness after training stops. Beneficial effects diminish within ~2 weeks of substantially reduced activity and can fully disappear within 2–8 months. |
| Double support |
The portion of a walking stride when both feet are on the ground simultaneously. |
| Duty factor (DF) |
Fraction of the stride cycle spent in stance; > 0.5 for walking, < 0.5 for aerial running. |
| Dynamometer |
A rigid device that fixes joint angle and measures resulting joint torque during a maximal contraction; the human equivalent of a muscle ergometer. |
| Eccentric contraction |
Lengthening contraction in which the load exceeds muscle force; performs negative work; most economic per unit force but greatest injury risk. |
| Effective mechanical advantage (EMA) |
Ratio of muscle moment arm to GRF moment arm (r/R) at a joint. Higher EMA → lower required muscle force per unit body weight. Scales positively with body mass across mammals and birds. |
| Endurance running hypothesis |
The proposal (Bramble & Lieberman 2004) that selection for endurance running shaped many unique anatomical features of Homo, with persistence hunting as a plausible behavioral context. |
| Endurance training |
Low-to-moderate intensity, high-repetition training that increases mitochondrial volume density and oxidative capacity across all fiber types. |
| Energy conservation (in MTU) |
Energy flow Body → Tendon → Body: muscle near-isometric, tendon cycles elastic energy. Underlies economy of steady-state gait (walking, running, hopping). |
| Evaporative cooling (sweat) |
A thermoregulation strategy via sweat evaporation from the skin. Humans have exceptionally many eccrine sweat glands and minimal body hair — a strong unambiguous specialization. |
| Excitation–contraction coupling |
The sequence linking sarcolemmal action potentials to Ca2+ release from the SR and, ultimately, cross-bridge activation. |
| Exercise-associated muscle cramps (EAMC) |
Painful, involuntary, sustained muscle contractions during or just after exercise. Best-supported mechanism is altered neuromuscular control (fatigue-driven hyperexcitability of α-motor neurons), not dehydration or electrolyte loss. |
| External tendon |
The free portion of the tendon that connects the muscle to bone (e.g., the visible Achilles tendon). |
| Factorial aerobic scope (fAS) |
Ratio of VO2 max to BMR. Athletic species sit on a separate scaling line 2–4× higher than non-athletic species at the same body size. |
| Fascicle / fiber length (Lfiber) |
The length of a muscle fascicle. Determines displacement and velocity capacity of the muscle (proportional to sarcomeres in series). |
| Fiber arrangement |
The geometric organization of muscle fibers within a muscle belly: parallel (longitudinal), pennate (unipennate, bipennate), or multipennate. |
| Flat running CoT |
The pattern observed in bipeds (humans and birds) of CoT changing little with running speed over a wide range. Unlike quadrupeds, bipeds commit to one bouncing gait across most running speeds. |
| Foot rolling |
The continuous translation of the center of pressure from heel to toe during stance, made possible by the foot’s curved geometry — reduces collision losses. |
| Force–length (length–tension) relationship |
The intrinsic relationship between muscle length and the maximum active isometric force it can produce; parabolic with a peak at the optimum length L0, explained by actin–myosin overlap. |
| Force–velocity (F–V) relationship |
The intrinsic hyperbolic relationship between shortening velocity and load; foundational result of Hill (1938). |
| Force platform |
An instrumented plate that measures the three components (vertical, fore-aft, medio-lateral) of GRF in real time. |
| Forefoot strike |
A running foot-strike pattern (ball of foot first) with smoother GRF rise; uses the foot arch as a spring; common in barefoot runners. |
| Fmax (P0) |
Maximum isometric force at zero velocity. |
| Gait variability |
Variation in stride parameters from step to step; increases on uneven terrain and raises CoT proportionally. |
| Generalist muscle |
A muscle of intermediate architecture that does modest positive work in steady gait, can upregulate for inclines, and provides stability on uneven terrain (e.g., guinea fowl gastrocnemius). |
| Graviportal posture |
The straight-legged, columnar limb posture seen in very large animals (elephants, rhinos), maximizing EMA to keep muscle forces tractable despite large body weight. |
| Groucho running |
Running with a deeply flexed knee and lowered center of mass; increases muscle force demand and metabolic cost (~50%); used experimentally to demonstrate the link between posture and energetics. |
| Ground reaction force (GRF) |
The force the ground exerts on the foot — equal and opposite to the force the foot exerts on the ground (Newton’s 3rd law). |
| Heater organ |
A muscle (e.g., extraocular in some tunas) that has lost contractile function and dedicates its calcium-cycling machinery to thermogenesis via futile Ca2+ cycling. |
| Hill-type muscle model |
A standard phenomenological muscle model based on the Hill F–V hyperbola, combined with an F–L curve and an activation factor. |
| Hominin |
Member of the human lineage after divergence from the chimpanzee lineage (~6 Mya). Includes Australopithecus, Homo erectus, Homo sapiens, etc. |
| Hovering flight |
Sustained flight in place; aerodynamically demands lift on both upstroke and downstroke, requiring extreme power output (e.g., hummingbirds). |
| Hoyt-Taylor rule |
Animals voluntarily choose speeds near the minimum of the U-shaped CoT curve within a gait and switch gaits near CoT-curve intersections. |
| Hybrid (co-expressing) fiber |
A muscle fiber that simultaneously expresses more than one myosin heavy-chain isoform (most commonly type IIa/IIx). Becomes more prevalent with age. |
| Hyperplasia |
Increase in muscle fiber number; observed in animal models but with limited evidence in humans. |
| Hypertrophy |
Increase in muscle fiber size (cross-sectional area) in response to training; allows simultaneous increases in absolute amounts of myofibrils, mitochondria, and SR. |
| Intermyofibrillar mitochondria |
Mitochondria distributed between myofibrils; supply ATP for cross-bridge cycling. |
| Intramuscular fat infiltration |
Adipose tissue accumulation within the muscle belly; increases with age and reduces force per unit muscle volume (muscle quality). |
| Inverse dynamics |
A method for inferring muscle and joint forces from external measurements (motion capture, ground reaction force) by sequentially applying lever-system analysis at each joint. |
| Inverted pendulum model |
The walking analogy in which the body vaults over a stiff stance leg, exchanging gravitational and kinetic energy out of phase. |
| Isokinetic contraction |
Contraction at constant velocity; produced experimentally by an isokinetic dynamometer. |
| Isometric contraction |
Constant-length contraction with force generation but no shortening; no mechanical work, but ATP cost is proportional to force. |
| Isometric scaling |
Geometric scaling in which all linear dimensions grow in proportion; under isometry, strength (∝ L²) grows slower than mass (∝ L³). |
| Isotonic contraction |
Contraction at constant force; an experimental condition used to isolate the F–V relationship via load clamps. |
| Lateral gastrocnemius (LG) |
A common ankle extensor used in classic in vivo studies (Roberts 1997 turkey, Daley 2003 guinea fowl). |
| LT/Lo ratio |
Ratio of tendon slack length to optimal muscle fiber length. High → economic force, elastic cycling. Low → range of motion and position control. |
| M-shaped vertical GRF |
Characteristic double-hump vertical force trace of walking, with peaks at early and late stance and a mid-stance trough. |
| Mass-spring model |
A point-mass body on a massless springy leg; reproduces GRF magnitudes and timing in bouncing gaits across diverse legged animals. |
| Maximum voluntary contraction (MVC) |
Largest force a person can produce voluntarily; less than the muscle’s true maximum because the nervous system imposes a safety factor. |
| Mechanical efficiency |
Mechanical work output divided by total energy expenditure; peaks at lower velocities than peak power. |
| Mitochondrial volume density |
Fraction of muscle cell volume occupied by mitochondria; increases with endurance training in all fiber types. |
| Motor (muscle function) |
A muscle that shortens while producing high force, generating substantial positive net work per cycle. |
| Motor unit |
The functional unit of muscle activation: a single motor neuron and all the muscle fibers it innervates. |
| mTOR |
Mammalian target of rapamycin; the kinase that initiates protein synthesis in response to resistance-training mechanoreceptor activation. |
| Multifunctional muscle |
A muscle whose architecture allows it to act as a strut on level ground and a motor on inclines or during acceleration. Turkey lateral gastrocnemius is the canonical example. |
| Muscle ergometer |
A laboratory device that controls and measures muscle length and force; the workhorse of in vitro muscle mechanics. |
| Muscle memory |
Faster regain of fitness on retraining than during the original training, attributed to retained myonuclei (cellular component) and DNA methylation patterns (epigenetic component). |
| Muscle moment arm (r) |
Perpendicular distance from the muscle’s line of action to the joint center; determined primarily by skeletal morphology. |
| Muscle quality |
Force-producing capacity per unit muscle volume; declines with age even when volume is preserved, due to intramuscular fat infiltration. |
| Muscle–tendon unit (MTU) |
The functional unit comprising the muscle belly plus its associated tendon and aponeurosis. |
| Myofibril |
A long, cylindrical chain of sarcomeres within a muscle fiber; many myofibrils together fill most of the fiber volume. |
| Myofiber (muscle fiber) |
A single multi-nucleated skeletal muscle cell, specialized for force generation and movement. |
| Myonucleus / myonuclear domain |
A nucleus within a multinucleated muscle fiber; each regulates a fixed domain of cytoplasm, so hypertrophy beyond a threshold requires recruiting additional myonuclei via satellite-cell fusion. |
| Myosin (thick filament) |
The thick contractile filament; its globular heads form cross-bridges with actin and undergo the power stroke. |
| Myosin heavy chain (MHC) isoforms |
The myosin protein variants that define fiber type in skeletal muscle (MHC-I, MHC-IIa, MHC-IIx). |
| Net joint work |
Work done at a joint over a cycle; positive → energy generation; negative → energy absorption; near-zero → spring-like function. |
| Nuchal ligament |
A passive elastic ligament at the back of the neck in humans and many cursorial mammals; helps stabilize the head against pitching during running. |
| Optimum length (L0) |
The fiber/muscle length at which active isometric force is maximum; corresponds to maximal actin–myosin overlap. |
| Oropharyngeal reflex |
Sensory pathway by which strong tastes (e.g., pickle juice) activate receptors that send inhibitory signals to spinal α-motor neurons — proposed mechanism for cramp relief. |
| Overload |
Physical stress greater than usual that elicits adaptive plasticity in the trained system. |
| Overtraining |
Progressive decline in performance when training stress exceeds recovery capacity. |
| Oxycaloric coefficient |
Energy released per unit oxygen consumed during aerobic metabolism; ~20.1 J/mL O2 on average. |
| Passive-dynamic walking |
Bipedal locomotion driven by gravity plus mechanical-system geometry with minimal actuation; demonstrated in McGeer-style passive walkers. |
| Pectoralis |
The downstroke flight muscle in birds; in hummingbirds, composed exclusively of type IIa fibers packed with giant mitochondria. |
| Pennation angle |
The angle between muscle fibers and the line of action of the muscle–tendon unit. Increasing pennation packs more fibers per unit volume. |
| Persistence hunting |
A hunting strategy in which hunters track prey at sustained running speeds in heat until the prey overheats; often cited as the selective pressure for human endurance-running adaptations. |
| PGC-1α |
Master transcriptional coactivator that drives mitochondrial biogenesis after endurance training. |
| Physiological cross-sectional area (PCSA) |
Cross-section of muscle perpendicular to the fibers, computed as volume / fiber length. Determines the muscle’s maximum force capacity (proportional to sarcomeres in parallel). |
| Plantar aponeurosis |
The passive elastic sheet from the calcaneus to the toe pads, supporting the longitudinal arch and acting as the foot’s principal series elastic element. |
| Plantar arch |
The longitudinal arch of the human foot, supported by the plantar aponeurosis; acts as a tunable spring that stores and returns elastic energy during running. |
| Plantigrade / digitigrade / unguligrade posture |
Three foot postures along a continuum of distal-limb elongation. Plantigrade (humans, bears) — flat foot; digitigrade (dogs, cats, birds) — toes on ground; unguligrade (horses, ungulates) — tips of toes on ground. |
| Pontzer’s economy and endurance studies |
Comparative studies showing that humans have dramatically greater endurance than chimpanzees, standard muscle-mass-scaled VO2 max, and exceptionally economical walking relative to other primates. |
| Posture shift with body size |
The trend across mammals for limb posture to become more upright with increasing body size, raising EMA so that muscle force can keep up with body weight. |
| Power amplification (in MTU) |
Energy flow Muscle → Tendon → Body: muscle slowly loads the tendon, which rapidly recoils to release energy at a higher rate than the muscle alone could produce. Underlies jumping and ballistic feeding. |
| Power attenuation (in MTU) |
Energy flow Body → Tendon → Muscle: tendon absorbs body energy, muscle dissipates it by lengthening under load. Underlies landing and decline running. |
| Power stroke |
The conformational change in the myosin head, triggered by Pi release, that ratchets the actin filament past myosin. |
| Power–velocity curve |
Power = F × V plotted across the F–V curve; rises from zero, peaks at an intermediate velocity (~0.2–0.3 Vmax), and falls to zero at Vmax. |
| Progression |
The need to continually increase the training stimulus once a fitness level is reached. |
| Proximo-distal gradient |
The pattern in cursorial limbs of placing high-mass power-producing muscles proximally and specialized short-fibered spring muscles distally; reduces distal-limb inertia. |
| Push-off |
Trailing-leg work that adds energy to the body; effective push-off just before heel strike reduces the upcoming collision. |
| Raichlen & Polk brain-evolution hypothesis |
The proposal that selection for endurance activity raised baseline neurotrophin/growth-factor signaling in early Homo, indirectly driving the evolution of large brain size and cognition. |
| Rear-foot strike |
A running foot-strike pattern (heel first) producing a small early impact peak in the vertical GRF; common in shod runners. |
| Reduced-gravity experiments |
Treadmill experiments with partial-weight suspension demonstrating that running metabolic cost is approximately proportional to gravity, while walking cost is largely insensitive — confirming that running is force-limited and walking is work-limited. |
| Regional endothermy |
Maintenance of elevated temperature in selected tissues using vascular countercurrent heat exchangers (e.g., red swimming muscle in tunas). |
| Resistance training |
High-intensity, low-rep training; increases type II fiber size and produces a small IIx → IIa fiber-type shift. |
| Rete mirabile |
A vascular countercurrent heat exchanger that traps metabolic heat in tissues such as red muscle of tunas and mackerel sharks. |
| Reversibility |
Loss of training-induced gains when training stops; cardiovascular adaptations decay fastest, peripheral and structural adaptations more slowly. |
| Rigor |
The state in which actin and myosin remain tightly bound because no ATP is available; underlies rigor mortis. |
| Rubenson et al. reappraisal |
Modern re-analysis showing that human running CoT is ~17% higher than expected for body mass, while walking CoT is ~20% lower — a stronger case for walking specialization than running. |
| Running blade prosthesis |
Carbon-fiber prosthetic foot designed to mimic the elastic energy cycling of the natural Achilles tendon; stiffness must be individualized. |
| Sarcolemma |
The plasma membrane of a muscle cell; specialized for action potential propagation. |
| Sarcomere |
The functional contractile unit of striated muscle, defined between two Z-discs; contains overlapping actin and myosin filaments. |
| Sarcopenia |
Age-related loss of muscle mass and strength; primarily reflects type II fiber atrophy with preserved type I fiber size. |
| Sarcoplasm |
The cytoplasm of a muscle cell; densely packed with myofibrils, mitochondria, and SR. |
| Sarcoplasmic reticulum (SR) |
Specialized intracellular network that stores Ca2+ and contains Ca2+-ATPase pumps; functionally analogous to the endoplasmic reticulum. |
| Satellite cell |
Resident muscle stem cell that proliferates and either fuses with fibers to donate new myonuclei or releases exosomes that regulate ECM remodeling and angiogenesis. |
| Sliding filament model |
The model that explains sarcomere shortening as the result of cross-bridge cycling sliding actin past myosin without changes in filament length. |
| Soleus |
A major human ankle extensor; in walking and running operates near-isometrically (spring-like) with most MTU length change taken up by the Achilles tendon. |
| Sonomicrometry |
An experimental technique using implanted piezoelectric crystals to measure fascicle length during in vivo contraction via ultrasonic time-of-flight. |
| Specific tension (σ) |
Force per unit physiological cross-sectional area; ~18–30 N/cm² across vertebrate skeletal muscle. |
| Specificity |
Adaptations are specific to the body systems, muscle groups, contraction types, velocities, and ranges of motion trained. |
| Spring specialist |
A muscle with extreme architecture (very short fibers, very long thin tendon) constrained to act primarily as part of an elastic spring — e.g., wallaby plantaris. Trade-off: low safety factor for tendon injury. |
| SR Ca2+-ATPase (SERCA) |
The pump that re-sequesters Ca2+ into the SR after activation; a major ATP consumer (~30–40% of isometric ATP cost). |
| Stance phase |
The portion of the stride cycle when the foot is on the ground. |
| Step-to-step transition |
The brief interval when the trailing leg pushes off and the leading leg collides; the primary site of energy loss in walking. |
| Stiff tendon |
A tendon that stretches little under load; transmits force directly to rotate the joint. High AT/PCSA. |
| Stroke volume (SV) |
Volume of blood ejected per heartbeat; the most rapidly trained — and most rapidly detrained — cardiovascular variable. |
| Strut (muscle function) |
A muscle that contracts near-isometrically during force development, allowing the tendon to act as a passive spring. Produces no net muscle work but transmits force to the skeleton. |
| Subsarcolemmal mitochondria |
Mitochondria packed beneath the sarcolemma; supply ATP for SR Ca2+-ATPase activity. |
| Supercompensation |
The recovery cycle in which fitness dips below baseline after overload, then rebounds above baseline during adaptation. |
| Superfast (sonic) muscle |
A specialized muscle (e.g., toadfish swim bladder, rattlesnake tail-shaker) with very high SR volume fraction (~30%) supporting contraction rates >100 Hz; trades myofibril volume for SR. |
| Supracoracoideus |
The avian upstroke flight muscle; routes through a tendon over the shoulder to lift the wing. |
| Tendon buckle |
A surgically implanted strain-gauge transducer that wraps around a tendon and converts tendon deformation into a force signal. |
| Tendon slack length (LT) |
The length of the tendon at zero force; one of the key architectural parameters. |
| Titin |
A giant elastic structural protein that maintains sarcomere alignment and contributes to passive tension. |
| Torpor |
A state of greatly reduced metabolic rate and body temperature; used by hummingbirds when ambient temperatures fall too low for muscle function. |
| Torque (T) |
Rotational effect of a force, $T = F \times D$; measured in newton-meters (N·m). |
| Trainability |
The magnitude of adaptive response to a given training stimulus; strongly influenced by genetics. Low responders may gain only 2–3% in VO2 max, while high responders can gain ~50%. |
| Triad |
The structural unit formed by one T-tubule and two flanking SR terminal cisternae; the site of excitation–contraction coupling. |
| Triceps surae |
The compound calf muscle group: lateral gastrocnemius, medial gastrocnemius, and soleus; inserts via the Achilles tendon. |
| Troponin |
A regulatory protein bound to actin/tropomyosin; binds Ca2+ and exposes actin binding sites for cross-bridge formation. |
| TSC1/2 |
Tuberous sclerosis complex; an inhibitor of mTOR activated by AMPK during endurance training. |
| T-tubule |
An invagination of the sarcolemma that conducts the action potential into the cell interior, contacting the SR at the triad. |
| Twitch |
The brief mechanical response to a single action potential; its time course depends on Ca2+ release/uptake and cross-bridge kinetics. |
| Type I fiber (slow oxidative) |
Slow, fatigue-resistant fiber with high mitochondrial density, low ATPase activity, and high efficiency. |
| Type IIa fiber (fast oxidative–glycolytic) |
Fast fiber with high mitochondrial density and intermediate fatigue resistance. |
| Type IIx fiber (fast glycolytic) |
Fast fiber with low mitochondrial density, high ATPase activity, high specific tension, and rapid fatigue. |
| Vmax (maximum shortening velocity) |
Maximum unloaded velocity at which a muscle fiber can shorten; primarily determined by myosin isoform; declines with body size across mammals. |
| VO2 max |
Maximal rate of oxygen consumption during exercise; the canonical metric of aerobic capacity. Average training improvement is 15–20%; ~50% of inter-individual variation is heritable. |
| Walk-to-run gait transition |
The speed (~2.0 m/s in humans) at which gait switches from walking to running; helps keep muscles like the soleus near their optimal F–L and F–V operating point. |
| Weyand step-cycle equation |
Equation predicting average vertical GRF (in body weights) from the ratio of step duration to stance duration. |
| Work loop |
Plot of muscle force vs. muscle length over a contraction cycle; enclosed area equals net mechanical work per cycle. Counterclockwise → motor; clockwise → brake; narrow/no area → strut/spring. |
| Z-disc (Z-line) |
The structural boundary of a sarcomere; anchors actin (thin) filaments. |
| Zero-sum game (volume fractions) |
The principle that a fixed muscle-cell volume must be partitioned among myofibrils, SR, and mitochondria, so increases in one come at the expense of the others (Rome & Lindstedt 1998). |